The solid polymer type fuel cell has such features as high output, long service life, less reduction of performance by start and stop, low operation temperature (about 70 to 80° C.), etc. Since the fuel cell has a feature that the start and stop of the fuel cell are easy, the fuel cell is expected to be used in a wide field including a power source of electric cars, distributed business power source, home power sources, etc.
Among the above usages, the distributed power source installing the polymer type fuel cell (such as cogeneration power generation system) produces electric power, and at the same time, heat generated at the time of power generation is recovered as hot water so as to utilize energy effectively. The distributed power source requires a service life of 50,000 hours or more; thus, development of membrane-electrode assemblies, cell structures, power generation conditions, etc has been conducted. As for the total system of the power generation system installing the fuel cell, the lowering of output and power generation efficiency by the repetition of stop and start should be as small as possible is wanted by the users. Particularly, it is known that at the time of sopping of the fuel cell, fuel gas is withdrawn from the fuel cell (burnable gas) so as to prevent lowering of the output voltage. As a relating art, in a phosphoric acid fuel cell system, a technology for stopping the fuel cell was known in which inert gas purge is utilized (Patent Publication 1). In order to save a space for the fuel cell system and downsize the system, a system that does not use the inert gas is desired. A method of stopping the solid polymer type fuel cell using a resister for short-circuiting the outer circuit is disclosed (Patent Publication 2).
When the fuel cell system installing the solid polymer fuel cell is stopped, there may be a case where a drop of output takes place by repeating start-stop of the system, i.e. start and stop of the solid polymer fuel cell, depending on a condition. Particularly, at the time of stopping the fuel cell, if the fuel cell voltage is maintained at 0.8 volt per unit cell or higher under a high temperature state, sintering of catalyst takes place to gather catalyst particles thereby to decrease an active surface area and lower a fuel cell voltage (Patent Publication 1). In order to avoid the problem, it is necessary to remove hydrogen and oxygen remaining in the fuel cell. According to the conventional technology, a method is disclosed wherein inert gas is supplied to the anode and cathode, and a short-circuiting the outer circuit by connecting a dummy resister and each of the cells is employed to stop the fuel cell.
As another system, there is a method wherein a switch for connecting a resister and the fuel cell is operated in accordance with an amount of air during an amount of air to be supplied to the cathode is not zero or in accordance with a output voltage of the fuel cell after the amount of air becomes zero. In the method, since oxygen of the cathode gives a larger affect on the fuel cell than does fuel, i.e. hydrogen, the control is done for stopping the fuel cell in considering only the amount of air.
Patent Publication 1; Japanese Patent Laid-open Hei 10-144334
Patent Publication 2; Japanese Patent Publication Shou 63-181268